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Extending Mendelian Genetics DO NOT EDIT--Changes must be made through “File info” CorrectionKey=B Extending CHAPTER 7 Mendelian Genetics BIG IDEA Mendel’s laws of inheritance do not account for the expression of all traits, which may be influenced by the number of genes involved, linkages with other genes, or the environment in which the organism lives. 7.1 Chromosomes and Phenotype 7.2 Complex Patterns of Inheritance 6F 7.3 Gene Linkage and Mapping 3F, 6H Data Analysis CONSTRUCTING BAR GRAPHS 2G 7.4 Human Genetics and Pedigrees 6F, 6H ONLINE BIOLOGY HMDScience.com ONLINE Labs ■■ Video Lab Plant Genetics ■■ QuickLab Sex-Linked Inheritance ■■ Codominance ■■ Pedigree Analysis ■■ Incomplete Dominance ■■ Human Heredity ■■ Examining Banding Patterns in Polytene Chromosomes (t) ©2006 Jupiterimages; (c) ©Diana Koenigsberg/Getty Images; (t) ©2006 Jupiterimages;(b) ©Gazimal/Getty Images (c) ©Diana Koenigsberg/Getty 190 Unit 3: Genetics DO NOT EDIT--Changes must be made through “File info” CorrectionKey=B Q Why are there so many variations among people? It will come as no surprise to you, but you are not a pea plant. But Mendel’s principles apply to you just as they apply to other organisms. About 99.9% of everyone’s DNA is identical. So how can a 0.1% difference in DNA lead to the wide range of human traits? In many organisms, genetics is more than dominant and recessive alleles. READING TOOLBOX This reading tool can help you learn the material in the following pages. USING LANGUAGE YOUR TURN Analogies An analogy question asks you to analyze the Use information in the chapter to complete this analogy. relationship between two words in one pair and to allele : gene :: trait: __________ identify a second pair of words that have the same (Hint: Finding out how alleles and genes are related will relationship. Colons are used to express the analogy for help you figure out which word to use to fill in the this type of question. For example, the analogy “up is to blank.) down as top is to bottom” is written “up : down :: top : bottom.” In this example, the relationship between the words in each pair is the same. (tl) ©Getty Images; (tc) ©Blend Images/Alamy; (tr) ©Kaz Mori/Getty Images; (cl) ©Getty Images; (cr) ©James Woodson/Getty Images; (bl) ©Nancy Honey/Getty Images; Images;(bc) ©Hans Neleman/Getty (bl) ©Nancy (br) ©Navaswan/GettyWoodson/Getty Images(tl) ©Getty Images; (tc) ©Blend Images/Alamy;©Getty Images; (tr) ©Kaz Mori/Getty Images;(cr) ©James (cl) Chapter 7: Extending Mendelian Genetics 191 DO NOT EDIT--Changes must be made through “File info” CorrectionKey=A 7.1 Chromosomes and Phenotype KeY COnCePt VOCABULARY The chromosomes on which genes are located can carrier affect the expression of traits. sex-linked gene MAIn IDeAS X chromosome inactivation Two copies of each autosomal gene affect phenotype. Males and females can differ in sex-linked traits. Connect to Your World The next time you are in a crowd of people, take a moment to look at the variety of traits around you. Hair color and texture, eye color and shape, height, and weight are all influenced by genetics. Can dominant and recessive alleles of one gene produce so many subtle differences in any of those traits? In most cases, the answer is no. But the dominant and recessive relationship among alleles is a good place to start when learning about the complexities of genetics. MAIn IDeA two copies of each autosomal gene affect phenotype. You have already learned how some genetic traits depend on dominant and recessive alleles. But many factors affect phenotype, including the specific chromosome upon which a gene is located. Gene expression is often related to whether a gene is located on an autosome or on a sex chromosome. Recall that sex chromosomes determine an organism’s sex. Autosomes are all of the other chromosomes, and they do not play a direct role in sex determination. You also know that sexually reproducing organisms have two of each chromosome. Each pair consists of one chromosome from each of two parents. Both chromosomes have the same genes, but the chromosomes might have different alleles for those genes. And, as Mendel observed, different alleles can produce different phenotypes, such as white flowers or purple flowers. All of the traits that Mendel studied are determined by genes on auto- somes. In fact, most traits in sexually reproducing organisms, including humans, are the result of autosomal genes. Look at Figure 1.1. Is your hair curly or straight? What about your parents’ hair? The genes that affect your hair texture—curly hair or straight hair—are autosomal genes. Many human genetic disorders are also caused by autosomal genes. The chance of a person having one of these disorders can be predicted, just as Mendel could predict the phenotypes that would appear in his pea plants. Figure 1.1 Hair texture is just one example of a trait that is Why? Because there are two copies of each gene on autosomes—one on each controlled by autosomal genes. homologous chromosome—and each copy can influence phenotype. ©Dimitri Vervits/Getty ©Dimitri Images 192 Unit 3: Genetics DO NOT EDIT--Changes must be made through “File info” CorrectionKey=A Disorders Caused by Recessive Alleles Figure 1.2 autosoMe InHeritAnCe Some human genetic disorders are caused by recessive alleles Some genetic disorders, such as cystic fibrosis, are on autosomes. Two copies of the recessive allele must be present inherited according to Mendel’s principles. for a person to have the disorder. These disorders often appear heterozygous parent (Cc), carrier in offspring of parents who are both heterozygotes. That is, each parent has one dominant, “normal” allele that masks the C c one disease-causing recessive allele. For example, cystic fibrosis is a severe recessive disorder that CC Cc C homozygous heterozygous, mainly affects the sweat glands and the mucus glands. A person dominant carrier who is homozygous for the recessive allele will have the disease. heterozygous parent (Cc), Someone who is heterozygous for the alleles will not have the carrier disease but is a carrier. A carrier does not show disease symp- Cc cc c heterozygous, homozygous toms but can pass on the disease-causing allele to offspring. In carrier recessive, aected this way, alleles that are lethal, or deadly, in a homozygous recessive individual can remain in a population’s gene pool. C 5 Normal allele (dominant) This inheritance pattern is shown in Figure 1.2. c 5 Cystic fibrosis allele (recessive) Disorders Caused by Dominant Alleles Dominant genetic disorders are far less common than recessive disorders. One example is Huntington’s disease. Huntington’s disease damages the nervous system and usually appears during adulthood. Because the disease is caused by a dominant allele, there is a 50% chance that a child will have it even if only one parent has one of the alleles. If both parents are heterozygous for the disease, there is a 75% chance that any of their children will inherit the disease. Because Huntington’s disease strikes later in life, a person with the allele can have children before the disease appears. In that way, the allele is passed on in the population even though the disease is fatal. Connect How are Mendel’s observations related to genes on autosomes? MAIn IDeA Males and females can differ in sex-linked traits. Mendel figured out much about heredity, but he did not know Figure 1.3 sex chromosome InHeritAnCe about chromosomes. As it turns out, he only studied traits produced by genes on autosomes. Now we know about sex The gametes from an XY male determine the sex chromosomes, and we know that the expression of genes on the of the offspring. sex chromosomes differs from the expression of autosomal genes. female parent (XX) XX Sex-Linked Genes Genes that are located on the sex chromosomes are called sex-linked genes. Recall that many species have specialized sex X XX XX female female chromosomes called the X and Y chromosomes. In mammals and some other animals, individuals with two X male parent (XY) chromosomes—an XX genotype—are female. Individuals with one X and one Y—an XY genotype—are male. As Figure 1.3 XY XY Y male male shows, a female can pass on only an X chromosome to offspring, but a male can pass on an X or a Y chromosome. Chapter 7: Extending Mendelian Genetics 193 DO NOT EDIT--Changes must be made through “File info” CorrectionKey=A R e ADI n G TO OLB ox Genes on the Y chromosome are responsible for the development of male offspring, but the X chromosome actually has much more influence over tAKInG nOTES phenotype. The X chromosome has many genes that affect many traits. Use a two-column chart to compare and contrast the Scientists hypothesize that the Y chromosome may have genes for more than expression of autosomal and sex determination, but there is little evidence to support this idea. sex-linked genes. In many organisms, including humans, the Y chromosome is much smaller autosomes sex chromosomes and has many fewer genes than the X chromosome. Evidence suggests that over millions of years of evolution, the joining of the X and Y chromosomes during meiosis has resulted in segments of the Y chromosome being trans- ferred to the X. You will read more about specific sex-linked genes and their locations on the human X and Y chromosomes in Section 4. expression of Sex-Linked Genes Because the X and Y chromosomes have different genes, sex-linked genes have a pattern of expression that is different from autosomal genes. Remember, two Biology VIDEO C copies of an autosomal gene affect a trait.
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